Stereoscopic display of visual scenes requires that a separate two-dimensional image be supplied to each eye of the observer, wherein each image is a member of a stereo pair that depicts the scene from a different perspective viewpoint. The observer's visual system fuses the images, providing a realistic three dimensional impression. Many known systems require the observer to wear glasses, goggles or similar devices to supply separate images to each eye. For many applications, however, such devices are inconvenient or impractical.
In contrast, an autostereoscopic three dimensional display directs a separate two-dimensional image to each eye of the observer without requiring the observer to wear any device. An optical system superimposes the two-dimensional images on a viewing plane, while limiting the viewing zone of each image to a separate exit pupil or virtual aperture in the vicinity of the observer's eyes. The virtual aperture size, shape and position are controlled so that each eye looks into a different virtual aperture and sees a different two-dimensional image. This approach works with as few as two virtual apertures and two views, but in this case the observer must seek out the position where each eye is looking through the correct virtual aperture. A side-by-side array of multiple virtual apertures provides more freedom for the observer to move around while continuing to view the scene. So long as each virtual aperture is narrower than the observer's interocular separation and each adjacent pair of virtual apertures shows stereoscopically related perspective views of the scene, the observer will perceive a realistic three dimensional impression of the scene when both eyes are positioned within the array. Further, when the observer moves side-to-side within the array, his eyes will transition from view to view and he will experience a life-like change in the scene perspective. A wide array of multiple virtual apertures also allows multiple observers to view the same scene. Alternatively, each observer may be shown a different scene in applications such as video games. In all cases the systems are very intuitive to use: the observers simply approach the screen and perceive a 3-D image when they are in position to look through the virtual aperture array.
The present invention relates to the class of autostereoscopic display systems in which multiple projectors, particularly video projectors, superimpose multiple perspective views of the scene onto either a transmissive rear projection screen or a reflective front projection optical screen that directs light to form an array of virtual apertures, wherein a different perspective view is visible through each virtual aperture, but invisible elsewhere in the viewing area. Each individual projector in such systems can be of common and readily available frame-rate, resolution and light output, since the multiple image formation task is distributed over all the projectors. In contrast, some prior art autostereoscopic systems rely on a single specialized display device with very high frame-rate or very high resolution to form the required multiple images. Ongoing reductions in the size, cost and energy consumption of commonly available video projectors, exemplified by picoprojectors small enough and low enough in cost to be integrated into cellular telephones, make autostereoscopic systems incorporating multiple projectors an increasingly attractive approach.
Prior art front and rear projection optical screens to form virtual apertures, however, are problematic. The projectors form superimposed real images on the screen, and the function of the screen is to redirect each point of projector light focused on the screen so that it is evenly distributed over the associated virtual aperture, and excluded from other areas. The most efficient approach is to direct all the light to the virtual apertures, while approaches that block light are less efficient and require higher output projectors. Screens comprising a Fresnel lens collect light efficiently from multiple projectors over the screen area, and direct the light from each projector to a separate small area. Each small area is a real image of the lens exit pupil that forms a “peephole” virtual aperture through which the projected image from the associated projector is visible, and the projected image is invisible elsewhere. The “peepholes” require exact eye positioning, and are therefore not acceptable for most applications. Anisotropic diffusion means such as a pair of lenticular screens have been proposed in the prior art to expand the virtual apertures in the horizontal and vertical directions to increase the range of eye positions in which the image is visible. This expanding means is reported to be effective in the vertical direction where a large and relatively imprecise amount of scattering is acceptable, but providing a small but well-controlled amount of dispersion in the horizontal direction appears to be more difficult since the Gaussian nature of the diffusion causes non-uniform illumination across the virtual apertures and excessive double imaging at the edges of the apertures.
Double-sided refractive lenticular screens with a transmissive diffusion layer in-between have been proposed for rear projection autostereoscopic systems. Precise alignment of the front and rear lenticular features is critical and reportedly difficult. Single-sided refractive lenticular screens backed by a reflective diffusion layer avoid the need for alignment, but show cross-talk between virtual apertures caused by scattering. Slit mask screens that block light to form virtual apertures have similar difficulties, with the added disadvantage of poor luminous efficiency.
Fresnel lens screens used with projector lens apertures enlarged in diameter to the point that adjacent lens aperture edges adjoin have been proposed to increase the width of the virtual apertures and minimize the requirement for expansion by horizontal diffusion. The virtual apertures are further expanded in the vertical direction using single-direction diffusers such as lenticular screens. Special lenses have been described to accomplish this. Alternatively, projectors with large but more conventional lenses may be staggered up and down to achieve a similar effect. Drawbacks include large and sometimes complex projector lenses.
A need clearly exists for a multiple projector autostereoscopic system incorporating front or rear projection optical screens that precisely and efficiently direct the projector light to the virtual aperture array without depending on diffusion, and that can accommodate small projector apertures.